U.S. patent application number 16/653414 was filed with the patent office on 2021-04-15 for forming a multicolor phosphor-converted led array.
This patent application is currently assigned to Lumileds LLC. The applicant listed for this patent is Lumileds LLC. Invention is credited to Emma DOHNER, Hisashi MASUI, Kentaro SHIMIZU.
Application Number | 20210111315 16/653414 |
Document ID | / |
Family ID | 1000004409553 |
Filed Date | 2021-04-15 |
![](/patent/app/20210111315/US20210111315A1-20210415-D00000.png)
![](/patent/app/20210111315/US20210111315A1-20210415-D00001.png)
![](/patent/app/20210111315/US20210111315A1-20210415-D00002.png)
![](/patent/app/20210111315/US20210111315A1-20210415-D00003.png)
![](/patent/app/20210111315/US20210111315A1-20210415-D00004.png)
United States Patent
Application |
20210111315 |
Kind Code |
A1 |
DOHNER; Emma ; et
al. |
April 15, 2021 |
FORMING A MULTICOLOR PHOSPHOR-CONVERTED LED ARRAY
Abstract
A phosphor carrier assembly includes a substrate, a thermal or
UV activated release adhesive, a layer containing a pixelated
phosphor array, and a partially cured or highly viscous adhesive.
The phosphor pixels on the carrier are typically all of the same
color. In formation of a phosphor converted LED array the phosphor
pixels on the carrier assembly are aligned with and placed in
contact with corresponding LED pixels in an array of pixelated LED
dice. Selected phosphor pixels on the carrier assembly may then be
attached to corresponding LED pixels, and released from the
substrate, by powering (activating) the corresponding LED pixels to
heat the selected phosphor pixel to a temperature that releases the
thermal release adhesive and that cures or partially cures the
adhesive on the selected phosphor pixels in contact with the
corresponding LED pixels.
Inventors: |
DOHNER; Emma; (Redwood City,
CA) ; SHIMIZU; Kentaro; (Sunnyvale, CA) ;
MASUI; Hisashi; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lumileds LLC |
San Jose |
CA |
US |
|
|
Assignee: |
Lumileds LLC
San Jose
CA
|
Family ID: |
1000004409553 |
Appl. No.: |
16/653414 |
Filed: |
October 15, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 2933/0041 20130101;
H01L 33/505 20130101; H01L 25/0753 20130101 |
International
Class: |
H01L 33/50 20060101
H01L033/50; H01L 25/075 20060101 H01L025/075 |
Claims
1-8. (canceled)
9. A method for manufacturing a phosphor converted LED array, the
method comprising: positioning an LED array comprising LED pixels
and a phosphor structure comprising phosphor pixels with the LED
pixels of the LED array aligned with and in contact with
corresponding ones of the phosphor pixels of the phosphor
structure; and electrically operating selected ones of the LED
pixels to emit light and to heat the corresponding phosphor pixels
to a temperature that bonds each of the selected LED pixels to the
corresponding phosphor pixel.
10. The method of claim 9, wherein the phosphor structure comprises
a substrate and surfaces of the phosphor pixels opposite from the
LED array are attached to the substrate by a thermally releasable
adhesive, and operating the selected ones of the LED pixels to emit
light heats the corresponding phosphor pixels to a temperature that
releases the corresponding phosphor pixels from the substrate.
11. The method of claim 10, comprising removing from the LED array
the phosphor structure substrate and phosphor pixels not released
from the substrate.
12. The method of claim 9, wherein the phosphor structure comprises
a substrate and surfaces of the phosphor pixels opposite from the
LED array are attached to the substrate by a UV releasable
adhesive, and operating the selected ones of the LED pixels to emit
UV light releases the corresponding phosphor pixels from the
substrate.
13. The method of claim 12, comprising removing from the LED array
the phosphor structure substrate and phosphor pixels not released
from the substrate.
14. The method of claim 9 wherein all of the phosphor structure
pixels of the phosphor structure are of a first color, the method
further comprising: positioning the LED array and a second phosphor
structure comprising phosphor pixels of a second color different
from the first color with LED pixels of the LED array that are not
bonded to a phosphor pixel of the first color aligned with and in
contact with corresponding ones of the phosphor pixels of the
second phosphor structure; and electrically operating selected ones
of the LED pixels to emit light and to heat their corresponding
phosphor pixels of the second phosphor structure to a temperature
that bonds each of the selected LED pixels to the corresponding
phosphor pixel of the second phosphor structure.
15. The method of claim 9 wherein: the phosphor structure comprises
a substrate, a releasable adhesive coating a surface of the
substrate, a phosphor layer disposed on the releasable adhesive
coating and attached by the releasable coating to the substrate,
and a thermally activated adhesive disposed on a surface of the
phosphor layer opposite from the releasable adhesive; and
electrically operating the selected ones of the LED pixels to emit
light heats the thermally activated adhesive disposed on the
corresponding phosphor pixels to a temperature that cures the
thermally activated adhesive to bond the selected LED pixels to the
corresponding phosphor pixels.
16. The method of claim 15 wherein electrically operating the
selected ones of the LED pixels to emit light heats the thermally
activated adhesive disposed on the corresponding phosphor pixels to
a temperature of 100 C to 150 C.
17. The method of claim 15, wherein the releasable adhesive is a
thermally releasable adhesive, and electrically operating the
selected ones of the LED pixels to emit light heats the thermally
releasable adhesive to a temperature that releases the
corresponding phosphor pixels from the substrate.
18. The method of claim 17, wherein electrically operating the
selected ones of the LED pixels to emit light heats the thermally
releasable adhesive to a temperature of 150 C to 220 C.
19. The method of claim 17, comprising removing from the LED array
the substrate and phosphor pixels not released from the
substrate.
20. (canceled)
21. The method of claim 15 wherein the thermally activated adhesive
includes a thermally activated silicone adhesive.
22. The method of claim 15 wherein the thermally activated adhesive
includes a thermally activated epoxy adhesive.
23. The method of claim 15 wherein the thermally activated adhesive
has a refractive index of 1.41 to 1.6.
24. The method of claim 15 wherein the substrate includes a PET
carrier.
25. The method of claim 9 wherein the phosphor pixels comprise
material selected from a group consisting of phosphor/silicone,
ceramic phosphor, phosphor in glass, and phosphor on glass.
26. The method of claim 10 wherein the substrate includes a PET
carrier.
27. The method of claim 10 wherein electrically operating the
selected ones of the LED pixels to emit light heats the thermally
releasable adhesive to a temperature of 150 C to 220 C.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to phosphor-converted light
emitting diodes.
BACKGROUND
[0002] Red-Green-Blue (RGB) microLED displays are expected to be
the next generation of display technology, due to superior image
quality, lower power consumption, and increased reliability.
Currently, several methods exist to form large arrays of
multi-color, micron-scale, and closely spaced pixels. One option is
to assemble each pixel from individual red, green, and blue LED
dice. Another option is to pattern red and green subpixels onto a
blue pixelated die, either via a stamp process, photolithography or
ink-jet printing.
SUMMARY
[0003] This disclosure describes a monolithic approach to
manufacturing a high density patterned or multi-color phosphor
converted LED array, without serial pick and place steps for every
single pixel. This avoids the significant attach accuracy issues of
serial pick and place, which leads to wide gaps between neighboring
pixels. Unlike stamp-based approaches, there is no additional
patterning step required before the attach process--color
selectivity occurs during phosphor integration. By using a
monolithic approach and a one-step per color attach, accuracy can
be enhanced. Additionally, yield improvements can be addressed by
incorporating a small number of error correction steps in the
phosphor placement process.
[0004] In the methods disclosed herein, a layered carrier assembly
includes, in order, a substrate, e.g. PET, a thermal or UV
activated release adhesive, a layer containing a segmented
(pixelated) phosphor array, and a partially cured or highly viscous
silicone adhesive. The phosphor pixels on a carrier are typically
all of the same color.
[0005] In formation of a phosphor converted LED array by these
methods, the phosphor pixels on the layered carrier assembly are
aligned with and placed in contact with corresponding LED pixels in
an array of pixelated LED dice, which may for example have a
Thin-Film Flip Chip (TFFC) architecture. Selected phosphor pixels
on the carrier assembly may then be attached to corresponding LED
pixels, and released from the substrate, by powering (activating)
the corresponding LED pixels to heat the selected phosphor pixel to
a desired temperature. This releases the thermal release adhesive
attaching the selected phosphor pixels to the carrier substrate and
cures or partially cures the adhesive on the selected phosphor
pixels in contact with the corresponding LED pixels. Thus, the step
of attaching selectively bonds pixels of the phosphor layer to
corresponding pixels of the array of pixelated dice. The layered
carrier assembly and un-bonded phosphor pixels may then be removed,
for example with a process tape attached (e.g., laminated) to a
backside of the substrate.
[0006] If a UV release adhesive is used instead of or in addition
to the thermal release adhesive, then UV light emitted by a GaN LED
may release the corresponding phosphor pixel from the carrier.
[0007] This process may be repeated with phosphor pixel arrays of
different color to produce a multicolour phosphor converted LED
array. The final device structure may be further heated such that
the silicone adhesive strongly bonds the phosphor pixels to the LED
pixels.
[0008] By using a pre-formed phosphor layer, the color control and
yield can be highly controlled by pre-testing and selecting only
known-good-phosphor for integration.
[0009] These and other embodiments, features and advantages of the
present invention will become more apparent to those skilled in the
art when taken with reference to the following more detailed
description of the invention in conjunction with the accompanying
drawings that are first briefly described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a cross-sectional schematic diagram of close
packed array of multi-colored phosphor converted LEDS on a
monolithic die and substrate.
[0011] FIG. 2 is a process flowchart according to the
invention.
[0012] FIG. 3 schematically illustrates a single color
pass--selective attachment of phosphor pixels of one
color--according to an example method of the invention.
[0013] FIG. 4 shows a top view of three pixelated LED dice to which
phosphor pixels have been selectively attached by methods as
described herein.
[0014] FIG. 5 schematically illustrates multiple color passes
according to an example method of the invention, in which phosphor
pixels of a first color are selectively attached to a pixelated
LED, and then phosphor pixels of another color are selectively
attached to the pixelated LED.
DETAILED DESCRIPTION
[0015] The following detailed description should be read with
reference to the drawings, in which identical reference numbers
refer to like elements throughout the different figures. The
drawings, which are not necessarily to scale, depict selective
embodiments and are not intended to limit the scope of the
invention. The detailed description illustrates by way of example,
not by way of limitation, the principles of the invention.
[0016] The term "GaN LED" is used herein to refer to III-Nitride
LEDs, i.e., to LEDs formed in the AlInGaN material system. The
examples below are describe with reference to such GaN LEDs, but
the methods described herein may be used with III-Phosphide
(AlInGaP material system) LEDs or with LEDs formed in any other
suitable material system.
[0017] FIG. 1 is a schematic cross-sectional view of a close packed
array 10 of multi-colored, phosphor converted LEDs on a monolithic
die and substrate 12, prepared by the methods disclosed herein. The
side view shows GaN LEDs 14 attached to the substrate 12 through
metal interconnects 16A (e.g., gold-gold interconnects or solder
attached to copper micropillars) and metal interconnects 16.
Phosphor pixels 18 are attached to corresponding GaN LEDs 14 though
an adhesive layer 20, which may be a silicone adhesive layer for
example. The pixels of the phosphor array 18 are coated on their
sides with a reflective mirror or diffusive scattering layer 22.
The phosphor pixels may be the same height for a single color or
more likely of varying heights in multi-color applications based on
the color point and sequential nature of the attach process, in
which case typically phosphor pixels of the last color deposited
will be the thickest (tallest). Optionally, a buffer layer or clear
layer 24 may be disposed on top of the phosphor array 18. This
helps to maintain the color point and also normalizes the thickness
or height variation on the phosphor layer 18. This also helps when
removing excess material applied as a reflective side coating to
the phosphor pixels.
[0018] As summarized above and further explained below with
reference to the figures, self-heating of the LED pixels and
corresponding phosphor pixels during operation of the LED dice is
used to selectively bond pixels from a phosphor array to
corresponding GaN LED pixels to form the phosphor converted LED
array.
[0019] FIG. 2 is a process flowchart for example methods of the
invention. FIG. 3 illustrates the device structure at various
stages of a single pass of phosphor pixel deposition by such
methods.
[0020] In step 102, a segmented phosphor array is created on a
carrier 30 coated with a releasable adhesive 32, e.g. a thermal
release layer. This array can be formed by dicing a layer that
contains phosphor, or by other methods such as a lithographic or
ablation process. The phosphor layer material may be, for example,
phosphor/silicone, ceramic phosphor, phosphor in glass, and
phosphor on glass. Before or after the phosphor layer segmentation
step, the surface of the phosphor layer (or of each phosphor pixel)
opposite from the surface attached to the carrier is coated with a
thermally-activated LED grade silicone or epoxy adhesive 20 (low
adhesion/G' at room temperature, increased adhesion at higher
temperatures).
[0021] The carrier may be for example a substrate of PET,
sacrificial polymer, glass, or silicon. The releasable adhesive
layer may be a thermal or UV activated release adhesive. The
thermal or UV release adhesive has a shear storage modulus
G'>300 KPa at room temperature, which drops below 300 KPa at
given elevated temperature to achieve adhesion with the carrier
substrate. Suitable layered carrier assemblies, e.g. Nitto Revalpha
and Adwill D510, include the substrate and thermal activated
release adhesive. The phosphor layer contains a segmented phosphor
array, and a partially cured or highly viscous silicone adhesive
disposed on a surface of the phosphor pixels opposite from the
carrier substrate.
[0022] During mechanical or chemical segmentation, the phosphor
layer may be diced into M.times.N arrays such that there are major
and minor gaps in the phosphor layer. The major gaps penetrate
fully through the carrier to singulate the array into pixels. The
minor gaps cut through phosphor, but stop in the carrier layer,
thus creating sub-pixels.
[0023] In step 104, the segmented phosphor array is aligned with
and placed in contact with an LED array so that individual phosphor
pixels are aligned with and optionally weakly bonded to
corresponding pixels in the array of LEDs.
[0024] In step 106, a selected subset of the pixels of the LED
array is electrically turned on. During this operation, at step 108
the surface of the selected LED pixels rapidly heats up, reaching
temperatures high enough (100-150 C) to cause the
thermally-activated adhesive between the die and phosphor pixels to
reflow, creating a strong bond between the selected die and
phosphor pixel. Simultaneously, down-conversion in the phosphor
layer results in additional heating of the phosphor pixel, reaching
temperatures of 150-200 C. This results in the release of that
phosphor pixel from the thermal release layer on the carrier.
[0025] In step 110, the carrier is removed. The initially selected
subset of pixels will be bonded to the LED (as shown in FIG. 3A),
while the rest will remain on the carrier.
[0026] FIG. 4 shows a top view of three 3.times.3 pixelated LED
dice to which phosphor pixels have been selectively attached by
methods as described herein. Metal interconnects 16 arranged on a
substrate allow individual LED pixels of each die to be separately
operated. Phosphor pixels 18 have been attached to underlying LED
pixels by operation of the LED pixels as described above. Exposed
GaN LED pixels 14 were not operated during the phosphor attachment
step.
[0027] As summarized above, steps 102-110 may be repeated with
different segmented phosphor arrays to yield a multicolored
phosphor-converted LED array. Here, the un-bonded phosphor pixels
remaining on the carrier form a patterned stamp, which can then be
bonded to a complimentary partial array of a different phosphor
color. FIG. 5 schematically illustrates an example of such
sequential deposition of two different colors of phosphor pixels by
the methods described herein. For color 1, steps 104-110 are
performed to deposit a pixel of color 1 on a first pixelated LED
die. For color 2, steps 104-110 are performed to deposit a pixel of
color 2 on a second pixelated LED die. After the initial deposition
step, the color 1 carrier has phosphor pixels and gaps arranged in
a manner that complements the position of the color 2 pixel on the
second pixelated LED die. Steps 104-110 may then be performed using
the color 1 carrier and the second pixelated LED die to selectively
deposit color 1 pixels on the second pixelated LED die, forming a
multicolour phosphor-converted LED array. A similar process may be
performed with the color 2 carrier and the first pixelated LED
die.
[0028] In one example, a silicone/phosphor film is fully cured and
then laminated onto a PET substrate coated with a thermal release
adhesive. A thin layer of silicone adhesive is deposited onto the
phosphor film surface via spin-coating, after which excess solvent
is removed with a short bake step. The phosphor film/PET stack is
then diced into 3.times.3 arrays, where every third saw line cuts
fully through the PET to singulate the arrays (other saw lines cut
through phosphor, but stop in PET layer). The arrays are then
aligned and attached onto a pixelated TFFC die using a
pick-and-place tool. No elevated temperature or force is used
during the attach process, leading to a weak bond between the die
and the silicone adhesive on the phosphor pixels. Electrical
contact is then made to power a selected pattern of die pixels,
which are run at a set current for a certain length of time (in the
case of devices shown in FIG. 4, 4A/mm.sup.2 for 15 seconds). A
tape is laminated to the top of the PET carrier and then used to
remove the PET carrier and un-bonded phosphor pixels.
[0029] Although the thermally-released carrier is removed at the
end of the attach process, this process requires a thermally
activated glue layer 20 between the phosphor layer and the die
surface. The presence of this glue layer and the fully aligned
segmentation of both the glue layer and phosphor layer will
indicate the use of this process. Other indicators of this method
may include the different heights of the phosphor layer depending
on the number of steps to integrate the various colors/pixel
arrays. Finally, the metal traces/tie bars used to electrically
activate the appropriate pixels during the attach process should
also be indicative of the use of this invention.
[0030] This disclosure is illustrative and not limiting. Further
modifications will be apparent to one skilled in the art in light
of this disclosure and are intended to fall within the scope of the
appended claims. One of skill can extend the disclosed invention to
vertical thin film (VTF) and similar LED architectures.
* * * * *